This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-110307, filed Apr. 9, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus for visualizing the nonlinear components generated by various nonlinear phenomena in a living body.
2. Description of the Related Art
When an ultrasonic wave propagates through tissues, the waveform is distorted by a nonlinear effect. As a result, a harmonic component is produced. When the ultrasonic wave strikes a microbubble (ultrasonic contrast agent), the microbubble nonlinearly vibrates and collapses. At this time as well, a harmonic component is generated. This mechanism of generating harmonic components is approximately given by a mathematical expression, for example the square of the amplitude of a fundamental frequency component. For this reason, harmonic components are effectively produced in a region with a high sound pressure. As compared with fundamental wave imaging, harmonic imaging has effects of, for example, narrowing transmitted beams and reducing sidelobes. Owing to such effects, images without artifacts can be generated.
In addition, the nonlinearity of an ultrasonic contrast agent is stronger than that of the living tissue. If, therefore, visualization is performed by extracting a harmonic component of a transmitted ultrasonic wave from a received echo, an image with a high contrast between the tissue and the contrast agent can be generated as compared with visualization with a fundamental wave.
A second harmonic is higher in frequency than the fundamental wave (transmitted ultrasonic wave), and hence is greatly influenced by frequency-dependent attenuation. FIG. 1 schematically shows changes in received echo with changes in depth due to the frequency-dependent attenuation in the living body. Referring to FIG. 1, reference numeral 11 denotes the spectrum of a received echo from a shallow region; 13, the spectrum of a received echo from a deep region; and 12, the spectrum of a received echo from an intermediate region. Obviously, with an increase in depth, the spectrum of a received echo shifts to the low frequency side. This phenomenon is a result of the characteristic that attenuation increases with an increase in frequency.
Owing to such a frequency-dependent attenuation characteristic, harmonic imaging has disadvantages, e.g., it is poor in penetration (the greatest depth that can be reached) as compared with fundamental imaging, and it is difficult to realize uniform image quality in the depth direction because the center frequency decreases (azimuth resolution deteriorates) and the band narrows (distance resolution deteriorates) with an increase in depth.
It is an object of the present invention to simultaneously realize narrowing of an ultrasonic beam, a reduction in sidelobe, an improvement in penetration, and uniformity of image quality in the depth direction.
An ultrasonic diagnostic apparatus according to an aspect of the present invention includes an ultrasonic probe, a transmitter for transmitting an ultrasonic wave pulse having peaks at fundamental frequencys, a receiver for receiving an echo signal corresponding the ultrasonic wave pulse, a filter configured to extract a difference frequency component by attenuating a fundamental frequency component centered on a fundamental frequency from the echo signal, and a processor configured to generate ultrasonic image data on the basis of the extracted difference frequency component.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.